ML021160585
| ML021160585 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 05/02/2002 |
| From: | Sheron B Office of Nuclear Reactor Regulation |
| To: | Macdonald D Pennsylvania State Univ, University Park, PA |
| Parczewski K, NRR/DE/EMCB, 415-2705 | |
| References | |
| YT #020020112 | |
| Download: ML021160585 (3) | |
Text
May 2, 2002 Professor Digby D. Macdonald Center for Electrochemical Science and Technology The Pennsylvania State University 201 Steidle Building University Park, PA 16802-5006
Subject:
Corrosion Damage to Reactor Pressure Vessel (RPV) Head at the Davis-Besse Nuclear Generating Station
Dear Professor Macdonald:
Thank you for sending us your paper dealing with determination of the pH of the concentrated boric acid solutions at elevated temperatures. The recent incident at the Davis-Besse Nuclear Generating Station underscored the importance of the damaging effects produced by leaking boric acid. While this event was unusual, occurrences of minor boric acid leakage have been reported at various times in different pressurized water reactors (PWRs).
The concentration of boric acid in the primary water of PWRs is usually around 2000 parts-per-million (ppm). At this concentration the solution is benign towards carbon steel components.
However, when it leaks out of the system and accumulates on the outside surface of a component, by losing some of its water by evaporation, it can produce a highly corrosive environment. The laboratory data have indicated that moist boric acid paste at 210F could corrode carbon steel at a rate of several hundred mils per year (Reference). Corrosion of carbon steel components by a leaking boric acid solution is, therefore, an important degradation mechanism and the NRC required licensees to have a special program dealing with this issue.
However, the current programs consist only of detecting boric acid leaks and repairing damaged components. They do not include determination of the rates at which corrosion is occurring. Since knowing the pH of boric acid deposits is an important factor in estimating corrosion rates, we have read your paper with great interest.
We fully appreciate the difficulties involved in measuring high temperature pH in the systems which, as you have indicated, the boric acid solutions are in some cases more a paste than a solution. Using the specially designed hydrogen and ceramic membrane electrodes and the Ag/AgCl, 0.1 m KCl external pressure balanced reference electrode and overcoming many experimental difficulties, you were able to determine pH of the concentrated solutions at temperatures ranging from 125 to 300C. Your finding of unusually low values of pH produced by the highly concentrated boric acid solutions at elevated temperatures can contribute to explaining the high corrosion rates of the boric acid deposit on the reactor vessel head. It should be recognized, however, that there are differences between the conditions chosen in your experimental work and those occurring at the reactor head in the Davis-Besse plant. Boric acid deposited on the reactor vessel head came from the leaking primary coolant and contained, therefore, very low concentration of other chemical species. The average molecular ratio of boron to lithium varied somewhere between 700 to 3000, depending on the regime of Digby D. Macdonald
coolant chemistry used. Also, the deposit of boric acid on the reactor vessel head was exposed to atmospheric oxygen. The data from your paper, although not directly applicable to the Davis-Besse case, provides much useful information. One of the most interesting findings is that there is a fair agreement between the values of pH calculated by the Electric Power Research Institutes (EPRIs) MULTEQ code and those determined in your experiments. Although the NRC does not possess the code, some licensees may be using it, and it is helpful to know that the Code is useful in making meaningful predictions.
We thank you for your interest in the boric acid corrosion issue at the Davis-Besse plant and for sharing your expertise.
Sincerely,
- original mailed to addressee 5/6/02 Brian W. Sheron, Associate Director /RA/
for Project Licensing and Technical Analysis Office of Nuclear Reactor Regulation
Reference:
C.J. Czajkowski, Corrosion and Stress Corrosion Cracking of Bolting Materials in Light Water Reactors, Proceedings of the International Symposium on Environmental Degradation of Materials in Nuclear Power Systems -
Water Reactors, August 1983.
Digby D. Macdonald 2
coolant chemistry used. Also, the deposit of boric acid on the reactor vessel head was exposed to atmospheric oxygen. The data from your paper, although not directly applicable to the Davis-Besse case, provides much useful information. One of the most interesting findings is that there is a fair agreement between the values of pH calculated by the Electric Power Research Institutes (EPRIs) MULTEQ code and those determined in your experiments. Although the NRC does not possess the code, some licensees may be using it, and it is helpful to know that the Code is useful in making meaningful predictions.
We thank you for your interest in the boric acid corrosion issue at the Davis-Besse plant and for sharing your expertise.
Sincerely, Brian W. Sheron, Associate Director for Project Licensing and Technical Analysis Office of Nuclear Reactor Regulation
Reference:
C.J. Czajkowski, Corrosion and Stress Corrosion Cracking of Bolting Materials in Light Water Reactors, Proceedings of the International Symposium on Environmental Degradation of Materials in Nuclear Power Systems -
Water Reactors, August 1983.
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